The present invention relates to compensation of imbalances among differential signals and in particular to a method and apparatus for providing compensated capacitance when the geometry of differential signal lines creates imbalance.
Whenever signals are conveyed from one electronic component to another over a physical medium such as a cable, there is a need to achieve at least some level of signal accuracy or integrity. For example, a certain level of signal accuracy or integrity is desirable for conveying signals from one network component, such as a first Ethernet node (e.g. an Ethernet hub or source) over a network cable to a second Ethernet node. Those of skill in the art will understand the need for a certain level of signal accuracy or integrity in other systems such as other networks, and any other variety of other digital and/or analog communication systems.
One aspect of communicated signals which is desirably maintained or enhanced is signal quality and/or relatively low levels of noise. In some situations, crosstalk noise is of particular concern. Accordingly, it would be useful to provide an apparatus, system and method which can make it feasible to enhance or maintain relatively high signal quality and/or reduce crosstalk or other noise.
Another potential problem with signal transmission is the level of immunity. In particular, some communication devices or system may be susceptible to interference from various external sources such as electrostatic discharge, radio broadcast signals and the like. Accordingly it would be useful to provide a signal communication apparatus, system and method which can enhance or maintain immunity, e.g., immunity to external sources such as electrostatic discharge, broadcast signals and the like.
In addition to concerns with signal quality, in many circumstances it is also desirable to reduce or avoid electromagnetic radiation from electronic components and/or cables. Accordingly, it would be useful to provide an apparatus system and method which can reduce or avoid the amount of emission from cables or electronic components.
In general, communication apparatus and systems are relatively cost-sensitive. For example, fabrication costs can include not only direct cost of obtaining and assembling components but also less-direct costs such as costs associated with occupying board space or volume (or other space or volume). Accordingly, it would be useful to provide an apparatus, system or method that can assist in achieving high signal quality and/or immunity at relatively low costs of design, fabrication, installation, maintenance and repair and/or operation.
The present invention includes a recognition of the existence and/or nature of certain problems, including as described herein. Although the present invention can be used in the context of a variety of different communication apparatus and systems, as one example, it has been found that a standard RJ-45 connector, e.g., for use in the context of an Ethernet network or communication system, can have an undesirably high degradation of signal quality or immunity. Without wishing to be bound by any theory, it is believed that at least some degradation of signal It quality or immunity is related to the non-symmetric pin locations in the RJ-45 connector used for the two wire pairs carrying differential signals, and thus creating a imbalance. Although, at least theoretically, signal degradation may be related to imbalances in any of a variety of electrical or electronic parameters (such as capacitance, inductance, or other parameters) without wishing to be bound by any theory, it is believed that at least some, and likely the greater part of, signal degradation in the context of systems such as Ethernet RJ-45 connections are associated with, or arise from, imbalances in capacitance of one or both conductors of a first differential pair with respect to a second differential pair. It is believed that imbalances can make such systems vulnerable to crosstalk or other noise and/or can lead to degradation of immunity to external noise sources.
Although it might be effective, in avoiding such noise problems, to use a different and more symmetric pin assignment scheme, it is believed this approach is substantially infeasible because of the large installed base of components using standard RJ-45 Ethernet (or other standard) pin assignment schemes.
It further would be possible to reduce or avoid at least some signal degradation potential by reconfiguring the cables, or the connectors by which cables are connected to circuit boards or other components. For example, wires or other signal carrying components in a connector or cable may be bent or moved (with respect to the configuration of traditional or standard cables or connectors) so as to modify capacitance or other electric or electronic parameters. However, in at least some embodiments of the present invention an approach involving modifying or reconfiguring cables or connectors is not preferred. Such an approach would place a burden on users to select and use such modified cables or connectors. Moreover, if a compensating cable or connector were used in conjunction with a compensating circuit board approach (e.g., as described below) the resultant overcompensation could increase noise levels.
According to one aspect of the invention, a circuit board is provided which achieves an amount of capacitive coupling between at least one conductor of a first differential signal pair (i.e. a pair of printed wires or traces used to convey a differential signal) and a second differential pair. Preferably the capacitive coupling is achieved without the need to use or mount discrete capacitors on the circuit board (which, it is believed, would undesirably increase fabrication or other costs). In one embodiment, a substantially conductive area or region is formed in or on the circuit board, aligned with, but spaced from, a portion of the second differential pair, with a conductive lead providing a conductive pathway from the region or area to at least one conductor of the first differential pair. In one embodiment, the conductive area or region and the second differential pair are formed on opposite surfaces of the circuit board such that the full thickness of the circuit board acts as a dielectric between the region or area and the second differential pair. In another embodiment, one or both of the portion of the differential pair and the conductive region or area are formed as internal layers in the circuit board, such that only a portion of the circuit board thickness acts as a dielectric. Preferably, the amount of capacitive coupling between the first conductor of the first differential pair and the second differential pair is sufficient to fully or partially compensate for the difference between the capacitive coupling (i.e., the capacitive coupling that would exist in the absence of the compensation) between the first conductor of the first pair (with respect to the second pair) and the second conductor of the first pair (with respect to the second pair).
In one aspect, a conductive area or layer of a circuit board is aligned with, and spaced from, a portion of a first differential signal conductor pair and connected through a conductive pathway to one of the conductors of a second differential signal conductor pair to provide capacitive coupling for compensating an imbalance arising from non-symmetric pin assignments. By compensating the imbalance, signal quality and/or immunity is improved and noise and/or emission is reduced.